treewide: replace error comparisons and assertions with errors.Is

Change-Id: Id2424eb155f2c6842c72c5fafd124d428ef901f2
Reviewed-on: https://review.monogon.dev/c/monogon/+/2994
Tested-by: Jenkins CI
Reviewed-by: Serge Bazanski <serge@monogon.tech>
diff --git a/metropolis/cli/metroctl/core/ca_tofu.go b/metropolis/cli/metroctl/core/ca_tofu.go
index fc30c6c..9df9957 100644
--- a/metropolis/cli/metroctl/core/ca_tofu.go
+++ b/metropolis/cli/metroctl/core/ca_tofu.go
@@ -114,7 +114,7 @@
 	if err == nil {
 		return ca, nil
 	}
-	if err != NoCACertificateError {
+	if !errors.Is(err, NoCACertificateError) {
 		return nil, err
 	}
 
diff --git a/metropolis/cli/metroctl/core/config.go b/metropolis/cli/metroctl/core/config.go
index 639bfc3..f3ca8b3 100644
--- a/metropolis/cli/metroctl/core/config.go
+++ b/metropolis/cli/metroctl/core/config.go
@@ -48,10 +48,10 @@
 // directory path, generating and saving it first if it doesn't exist.
 func GetOrMakeOwnerKey(path string) (ed25519.PrivateKey, error) {
 	existing, err := GetOwnerKey(path)
-	switch err {
-	case nil:
+	switch {
+	case err == nil:
 		return existing, nil
-	case NoCredentialsError:
+	case errors.Is(err, NoCredentialsError):
 	default:
 		return nil, err
 	}